The present invention relates to a spinal fixation system, and in particular to a fastener assembly for securing a longitudinal support along a spinal column.
1. Background of Invention
Stabilization of the spine is often required following trauma, tumor, or degenerative pathologies. Each region of the spine presents unique clinical challenges as several vital neural and vascular structures including the vertebral arteries, nerve roots, and spinal cord must be avoided during surgery. The anatomy of pediatric and small-statured patients presents additional challenges that makes assisting such patients even more difficult. For example, because these patients are small in stature, lower profile systems are required. Also, it may be required to add fastener assemblies to an already assembled system, especially in pediatric patients. At the same time, it is desirous to keep inventory at a minimum and have an assembly that requires as few components as possible.
Current methods of spinal fixation are not particularly well-suited for smaller statured patients. U.S. Pat. No. 5,737,685 to Hahn et al. discloses a bone screw that has a threaded shaft and a fork head. The fork head has two legs which define a single groove that opens in the vertical direction for receiving a corrective pin. The upper end of the fork head has an outside thread and a head nut is screwed onto the outer thread. The head nut has inner threads which a fastening screw is screwed to press onto the corrective pin. Because the groove opens in a vertical direction it is difficult to add additional screws to an already installed fixation system.
U.S. Pat. No. 5,530,441 to Sherman et al. discloses an attachment plate configured for use with a spinal fixation element that has a posteriorly projecting central post. The attachment plate is generally L-shaped with an eyebolt engaging portion and a fixation element clamping portion. The fixation element has grooves in the lateral surfaces of the central post and one lateral surface of the post contacts the spinal rod when the rod extends through an aperture of an eyebolt assembly. A flange extends from the attachment plate and is configured to engage the opposite lateral surface of the post and the attachment plate includes a camming segment to provide a clamping force component directed toward the flange to clamp the spinal rod to the fixation element post when a nut is threaded onto the eyebolt threaded post. The attachment plate has a slot in the eyebolt engaging portion for receiving the posteriorly projecting threaded post of the eyebolt body. Because the eyebolt must be pre-assembled onto the spinal rod, it is difficult to add additional fixation elements to an already installed system.
U.S. Pat. No. 4,653,481 to Howland et al. discloses a spinal support system that includes a plurality of screw clamp assemblies. Each screw clamp assembly has a threaded end for placement into the vertebra and a saddle assembly removably attached to the screw formed of upper and lower halves. Each of the upper and lower halves of the saddle assembly are provided with at least one pair of mating grooves for accepting a spinal rod. All embodiments shown and described in the Howland ""481 patent have a two-part saddle assembly that is removable from the screw clamp shaft member.
As the discussion above illustrates, there is a need for an improved fixation apparatus for stabilizing the spine that can be assembled to an already installed fixation system and has minimal components.
2. Summary of Invention
The present invention relates to a fastener assembly for a spinal fixation system. The fastener assembly includes a fastener, an attachment member, and a locking member. The fastener has a lower portion for contacting a bone and a upper portion integral with the lower portion and having two open channels. Each channel is configured and dimensioned for receiving a portion of the longitudinal member along its circumference. The attachment member is positionable on the fastener and at least partially covers the channel that receives the longitudinal member. The attachment member is configured and dimensioned for receiving another portion of the longitudinal member along its circumference. The locking member is operatively associated with the upper portion of the fastener and secures the attachment member and longitudinal member to the fastener.
The fastener can be a hook or a screw with the lower portion having a threaded end for engaging a vertebra. If the fastener is a hook, the hook can be provided with an arcuate portion and a flat portion for facilitating implantation. The arcuate portion can have a dimple on a posterior surface to further facilitate implantation. In one embodiment, the locking member is a nut and the upper portion of the fastener has a shaft with external threads to accept the locking member.
The two channels of the fastener are preferably disposed on opposite sides of a central plane of the fastener with the two channels extending orthogonally with respect to the fastener longitudinal axis and equidistant from the proximal end of the fastener. A bore can be positioned transversely to the longitudinal axis and between the two channels.
In an exemplary embodiment, the attachment member includes a cylinder having upper, lower, and side surfaces with a bore extending through the upper and lower surfaces and defining a longitudinal axis lying in a central plane; a slot extending through the cylinder offset from the central plane and parallel with the central plane; and a protrusion extending from the bottom surface on an opposite side of the central plane from the slot. The channels can define a seat for accepting the protrusion of the attachment member. The slot can be provided with serrations along the inner surface. The slot can also have an eccentric cross-sectional shape with a geometry substantially conforming to a diameter of the longitudinal member.
The fastener can have a cavity extending longitudinally from the distal end for receiving a manipulation device. This manipulation device includes a ball detent mechanism and a groove extending transversely to the cavity and around its circumference for receiving the ball detent mechanism. Alternatively, the manipulation device has a threaded end and the cavity of the fastener is cylindrical and includes threads along the interior.